Transmission circuit



May 10, 1932.V H. wHlTTLE TRANSMISSION CIRCUIT Filed Aug. 14, 1930INVENTOR ff. W/-l/TT/.E By

A TTORNE V Patented May 10, 1932 UNETED STATES PATENT OFFICE HORACEWHITTLE, OF MAPLEWOOD, NEW JERSEY, yASSIG-NOR TO BELL TELEPHONELABORATORIES, INCORPORATED, OF NEW YORK,v N. Y., .A CORPORATION 0F NEWYORK TRANSMISSION CIRCUIT This invention relates to transmissioncircuits and pertains more especially to the reduction of crosstallr inspeech transmission circuits in which repeating coils are employed forproviding phantom circuits.

It is an object of this invention to provide means and methods foreliminating the crosstalk in phantom circuits in which repeating coilsare employed. l

The repeating coils or transformers employed for this purpose shouldhave their windings as symmetrical with respect to the tapping point asis possible in order to reduce crosstalk to a minimum. A

Among the factors contributing to crosstalk between the phantom andkside circuits are inductance unbalance between the two halves of thewinding to which the ,phantom circuit is physically connected,resistance unbalance between the two halves of the winding to which thephantom circuit is physically connected, distributed capacity unbalancebetween the two halves of the winding to which the phantom circuit isphysically connected, mutual impedance imbalance between the windings ofthe repeating coil and admittance unbalance between the windings of therepeating coil.

The first four causes of crosstalk referred to above can be eliminatedby methods well known in the prior art, such as twisted pair windingsandbalancing resistances and cavpacitances.

The fifth cause of crosstalk referred to above is due to the capacitywhich exists between the primary or drop winding and the winding whichforms a part of the phantom lcircuit or line winding. In a coil of thistype there is a capacity between one-half of the line winding andone-half o f the primary or drop winding, a. second capacity between thesame half of the line winding and the other half of the primary or kdropwinding, a third capacity between the other half of the line winding andthe iirst mentioned half of the pl'lnary'or drop winding, and a fourthcapacity between the last mentioned half of the line winding andthesecond mentioned half of the primary or drop winding. Since the twohalves ofthe line windingare formed by a twisted pair, the lirst andthird' capaci i ities mentioned will be practically equal and the secondand fourth rcapacities mentioned will be practically equal. However,since the two halves of the primary or drop winding are wound onopposite halves of the core, the sum of the first and third capacitieswill not necessarily be equal to the sum of the second and fourthcapacities. When the sum of the first and third capacities is differentfrom the sum of the second and fourth capacities, the result is aneffective residual capacity, equal to the diference'between the twosums, acting vfrom the primary or drop winding to the line winding.

This effective residual capacity will in effect introduce a current inthe phantom circuit when a currentA flows in the side circuit. Theeffective current flowing in the phantom circuit will be proportional tothe magnitude of the residual capacity and the difference of potentialbetween its terminals. All the effective current flowing throughresidual capacity will not iow into the phantom circuit through thephysical connection to the side circuit, but this current will bedivided between the three'conductors leading from the line Winding ofthe repeating coil'. The amount of the effective current iiowing to theside circuit and the amount flowing to the phantom circuit will dependupon the relative impedance characteristics of the different paths.

It isan 'object of this' invention to eliminate the crosstalk in phantomcircuits due to the admittance imbalance between the windingsof therepeating coil.

If a small variable condenser of any suitl able typeV is introducedbetween two points that have a difference in potential opposite to thatof the residual capacity and the capacity of this condenser is adjusted,the crosstalk can be be balanced out or eliminated.

A feature ofthe invention is, therefore, the v tralize the admittanceunha-lance between the i the windings of each individual repeat-ingcoil, the repeating coil is preferably connected in a bridge ormeasuring circuit having balanced ratio arms and with the ratio armsungrounded the coil is kadjusted for self and mutual impedance andresistance unbalance. Then the current has been reduced to a minimum ina receiver connected from the tapping point, or point of physical lcon-10 i nection of the'phantoin circuit-to the point of connection ofbalanced ratio arms, it indicates that these unbalances are eliminated.lVith the coil connected in the bridge circuit in this manner noadmittance currents can flow due to the eiective residualcapacitybetween the windings as there is .no return path for these currents. The.mid-point of the .balanced yratio arm is then grounded which provides areturn path to the grounded source causing current to. flow through theeffective residual capacity between therwi-ndings. rI`he small variablecondenser isthen introduced between .two points in the coil which have adiiierence of potential opposite to that of the capacity unbalanceandthis condenser is adjusted un- `til the -crosstalk isbalanced out oreliminated.

As this condenser can in most cases be very small, it is preferably`formed by twisting two insulated conductors together, or by windingbare wire around an insulated conductor and soldering Athe turns of thebare wire together :to form a sheath, althoughany lsuitable variablecapacitance will be satisfactory for vthis purpose. T he capacity can bevaried .changing the .length of Wire twisted together .or the length of:the sheath `formed bythe bare wire. The advantage in `employing thistype of condenser is .that it can be firmly secured to the coil .afteradjustment.

The invention will vbe better understood by reference .totheaccompanying drawings and detailed description thereof, in which:

Fig. lis a-schematic `drawing or a phantom repeating coil;

Fig. 2 isa schematic drawing of a phantom circuit employing repeatingcoils of the type shown in Fig. 1; and

c Fig.r 3 is a schematic drawing of a testing ofcore structures, as forexample a toroidal core without an airgap and the so-called shell typecore with or without an airgap. TheV line winding 13, 14, 17, 18, ispreferably wound adjacent the core and the primary or drop winding 11,12, 15, 16 over the line winding. The line winding 13, 14, 17, 18comprises two windings 13, 14 and 17, 18 connected in series aiding. Thewinding 13, 14 comprises windings 13, 7 and 8, 14 connected in seriesaiding, and the winding 17, 18 comprises two windings 17, 9 and 10, 18connected in series aiding. findings 13, 7 and 17 9 comprise a .twistedpair of conductors wound on one-half of the vcore 2() and windings 8, 14and 17 9 comprise a twisted pair of conductors wound on the other halfof the core 20. The primary or drop winding 11, 16 comprises twowindings 11, 12 and 15, 16-ccnnected in series aiding. The winding 11,12 is wound over the windings 13, 7 and 17 9. The winding 15, 16 iswound over the windings 14, 8 and 18, 10. A variable condenser 5 isconnected from terminal 17', 14 to the terminal 11, to accomplishcapacity balance in accordance with the invention.

Referring to Fig. 2, a phantom repeating coil 24, similar inconstruction to that shown in Fig. 1 and described above has its primaryor drop winding 11, 12, 15, 16, connected to a side circuit 30, 31 andits line winding 13, 1.4, 17, 18 connected to line wires 32 and 33. Aphantom repeating coil 25, similar to the repeating coil shown in Fig.1, connects the other end of line wires 32 and 33 to a side circuit at34, '35. A phantom repeating coil 26, similar to the repeating coilshown in Fig. 1, has its primary or drop winding connected to a sidecircuit at 3:6, 37 and its line winding connectedto line wires 38 and39. A phantom repeating coil 27, similar to the repeating coil shown inFig. 1, connects the other end of the line wires 38 and 39 to anotherside circuit at 40, 41.

The primary winding of a repeating coil 28 is connected to a phantomcircuit at 42, 43. The secondary winding of the repeating coil 28isfconnected between the mid-point of the line winding 1.3, 14, 17, 18of the phantom repeating coil 24 at 14, 17 and the mid-point l of theline winding of the phantom repeating coil 26. The repeating coil 29connects the mid-point of the line windings of the phantom repeatingcoils 25 and 27 to a phantom l circuit at 44, 45.

Referring to the phantom repeating coil 24, of Fig. 2, constructed asshown in Fig. 1, it will be seen that there are eiiective capacities 1,2, 3 and 4 between the respective winding sections as shown.

Since the two halves 13, 7, 8, 14 and 17 9, 10, 18 of the line winding13, 14, 17 18 are formed by a .twisted pair of conductors, the capacity1 and the capacity 3 will be equal to each other and the capacity 2 andthe capacity 4 will be equal to each other. However, since the twohalves 11, 12, and 15, 16

of the primary or drop winding 11, 12, 15, 16

.are wound on opposite'sidesof the core 20 the sum of the capacities 1and 3 will not neceslim() 'iso sarily equal the suni of the capacities 2and 4. When the sum of the capacities 1 and 3 differs from the sum ofthe capacities 2 and 4 the result is an effective residual capacity,equal to the difference of the two sums acting from the primary or dropwinding 11, 12, 15, 16 to the line winding 13, 14, 17, 18.

There will then be either more or less current flowing from the sidecircuit through capacities 3 and 4 to the line winding than throughcapacities l and 2. All of the current flowing from the side circuitthrough the Vcapacities 1 to 4 inclusive will not tend to fiow in thephantom circuit through the physical connection of the repeating coil 28t0 the line winding 13, 14, 17 18, but will tend to divide between threeconductors, the one conductor to the line winding 13, 14, 17 18 at l14,17 and the other two conductors 32 and 33 leading from the line Winding.The amount of current tending to flow to the phantom circuit repeatingcoil 28 will'depend upon the relative impedancel characteristics of thethree conductor paths. If capacities 1 and 2 tend to cause a current L 2to fiow in the phantom circuit and the capacities 3 and 4 tend to causea current I3.,E to flow in the phantom circuit, the residual current Igflowing in the phantom circuit will be equal to lpg-134. The variablecapacity element 5 is provided by the present invention and is connectedfrom terminals 14, 17 to terminal l1 to produce a current I5 equal andopposite to the current IX.

Referring to Fig. 3, a phantom repeating coil constructed as shown in Fig. 1 and described above has its primary winding 11, 12, 15, 16connected to a source of constant frequency 46 through a repeating coil47, the mid-point of its secondary winding being connected to ground.Two resistances 48 and 49 having the same impedance characteristic areconnected in series at 50 and across the line winding 13, 14, 17, 18. Avariable resistance 51 and a receiver 52 are connected in series fromthe connection 50 of i'esistaiices 48 and 49 to the connection 14, 17 ofthe line winding13, 14, 17, 18. A switch 53 is connected from theconnection 50 of resistances 48, 49 and 51 to ground at 54.

The operation of this circuit to test the coil, for instance, forunbalance and to adjust it so as to neutralize the current flowing inthe phantom circuit due to the effective residual capacity is asfollows: l/Vith the switch 53 open the phantom repeating coil is firstadj usted for self and mutual impedances and resistance imbalance in themanner welllniown in the art and which is indicated by minimum currentflow inthe receiver 52.

With the phantom repeating coil connected in the circuit as shown andwith the switch 53 open, no admittance current can iiow from the winding11, 12, 15, 16 to the winding 13,

5 14, 17 18 due to the effective residual capacity between the windingsas there is no return path for these currents. Switch 53 is closed,providing a returnv path to the. grounded source 47, which causes acurrent Ix to flow from the grounded frequency source through i Thebalancing capacity provided byfthe present invention is then connectedfrom connectionf14, 17 on the line winding to a point on the priinaryordrop winding 11, 12, 15, 16 and is made to have such value as willproduce acurrent L between the connection 14, 17 on the line winding andground which is equal and opposite to the current Ix. (The condenser 5is preferably connected from 14, 17 on the line winding to either 11lor`16 on the drop winding.) The capacity of condenser 5 is then varieduntilno current flows in the receiver 52. This-completes the adjustmentof the phantom repeating coil for the elimination of admittanceimbalancel stalled in its permanent position in the phantom circuit andthe frequency source 46 and repeating coil 47 located in the centralcnice,

in which case any capacity unbalanceto ground `from the Ytwo sides ofthe line connecting thercentral office to the winding 11, 12, 15, 16willbe compensated for as will the residual capacitybetween the windingsof the coil.

It is to be understood that the invention is not limited to the'specilicembodimentV that has been shown and described ink detail, but only bythe scope of the appended claims.

lVhatis claimed is: f Y

1. r1`he method of eliminating admittance uiibalance due to the residualcapacity unbalanee between one winding of a repeating `coil and anotherwinding of said repeatingu coil symmetrical to a center tap, whichcomvided inner winding on said core wound in a plurality of layers, adivided outer winding wound in a plurality of layers over said innerwinding, said transformer having natural capacities between each half ofthe outer winding and each of the halves ofthe inner winding, whichcapacities are unsymmetrically located with respect to the commonterminals of the halves of the inner winding, and means comprisingaicapacity ele-ment connected between said common terminal of'thekhalves of the innerl winding,

andsaid vouter winding for compensating for said unsymmetrical locationof said capacities from the halves of the outer winding with respect tothe common terminal of the halves of the inner winding.

3. In combination, side and phantom circuits,a repeating coil in one ofsaid side circuits having .a divided primary and a divided secondarywinding, said windings so related that there eXists natural .capacitiesbetween each half of the primary winding and each of the halves of thesecondary winding, and that the capacit-ies betweeneach halof theprimary and the two halves of the secondary are equal, but the sum ofthe capacities between the one half of the primary and the two halves ofthe secondary diHers from the sum of the capacities between the otherhalf of the primary and thetwo halves of the secondary so as to producean admittance imbalance current between said windings, and a capacityelement connected between said primary and secondary winding whichneut-ralizes said admittance imbalance current.

4.. In combination, a transformer having primary and secondary windingsand natural capacities between said windings, said secondary windinghaving a center tap, a source ofcurrent connected to said primarywinding, an electrical conductor, one end of which is connected to saidcenter tap, an electrical path from the other end of said electricalconductor to said source of current, and means to prevent admittancecurrents from being transmitted from said primary winding to said sourceof current over an electrical path comprising said natural capacities,said electrical conductor, and said first mentioned electrical path,vsaid means comprising a capacity element connected between saidwindings.

'5. In combination, two lines, a phantom circuit, a repeating coilassociated with each of said lines and having a primary winding and acenter tapped secondary winding to mary winding to saidscurce of currentover an electrical path comprising said natural capacities, saidelectrical conductor, and said Vfirst mentioned electrical path, saidmeans comprising a capacity element connected between points in saidwindings which have a potential opposite to that of the eective residualcapacity due to said natural capacities between said primary andsecondary windings.

In witness whereof I hereunto subscribe` my name this 13th day ofAugust, 1930.

` HORACE WHITTLE.

which tap the phantom circuit is physically i connected, and means forbalancing out the admittance currents in each of said repeating coilsdue to the eifective residual capacity vunbalance between the windingsthereof, said means comprising an auxiliary capacity element connectedbetween points in lsaid windings which have a potential opposite to thatacross said residual capacity.

6. The method of eliminating admittance unbalance currents from arepeating coil having two windings, at least one of which windings issymmetrical to a center tap, which comprises adjusting said coil forself and mutual impedance and resistance unbalance, testing the coil foradmittance currents due to residual capacity unbalance between thewindings, adding a capacity element between points in said windingswhich have .a potential opposite to that across said re-

